A duplexer of a base station transceiver is formed by a radio frequency cavity filter, where the radio frequency cavity filter is generally located on a back mechanical part of a board of the transceiver and is configured to transmit a single-channel high-power signal. Due to an effect of a material thermal expansion characteristic, a filtering characteristic of the filter also varies with a temperature change. Particularly, the temperature has an extraordinarily prominent effect on a filtering characteristic of a narrowband cavity filter. Generally, a change of the temperature brings about a frequency band drift to a radio frequency index, commonly known as “temperature drift”, which causes a decrease in functions of a radio frequency system. Moreover, as mobile communications evolve to a high frequency band, the temperature drift phenomenon becomes increasingly serious, for example, for a cavity filter in a worldwide interoperability for microwave access (Worldwide Interoperability for Microwave Access, “WiMAX” for short) 2.6 GHz or 3.5 GHz standard, the frequency band drift phenomenon brought about by the change of temperature to the cavity filter has been very serious. A metal resonance tube manufactured by adopting a conventional aluminum alloy die casting and machining has difficulties to meet requirements of high-speed development of the communications technologies for the radio frequency index, which has been a main reason for hindering development of the high frequency band cavity filter.
By studying a relationship between a frequency of a cavity filter and a change of the temperature, it may be found that, each component dimension of a resonance tube in the cavity filter, for example, a width or diameter of a tuning screw, a width or diameter of a cavity, or a diameter or height of the resonance tube, may cause a change to the single cavity resonance frequency of the resonance tube or filter. Moreover, different component dimensions have different effects on the frequency of the filter when the temperature changes, for example, when the temperature rises, the height of the cavity causes a frequency change trend of the filter, in which the frequency change trend is quite the opposite to that caused by a height of a tuning rod. Therefore, temperature compensation may be performed on the cavity filter by using the characteristic.
Experimental studies show that, for a cavity filter without temperature compensation, when the temperature is +25° C., a central frequency of the filter is 2.4 GHz, while when the temperature changes to −40° C., the central frequency of the filter offsets to 2.4035 GHz, and the frequency offset is 3.5 MHz. Therefore, for the cavity filter without temperature compensation, when the temperature changes, a passband of the filter offsets, so at edge frequency points of a use frequency, an insertion loss is very high, and out-of-band rejection becomes worse, thereby directly causing deterioration of electrical properties of the filter and a decrease in system performance of the transceiver.
For a cavity filter on which temperature compensation is performed through the foregoing method, when the temperature changes from −40° C. to +25° C., the frequency variation of the filter may be less than 0.1 MHz, and a zero temperature drift may almost be implemented, thereby guaranteeing that the electrical properties of the cavity filter do not change at different temperatures.
By changing a component dimension of a cavity filter, temperature compensation may be performed on the cavity filter, but the changed component dimension may affect a Q value (quality factor) of the cavity. When the cavity dimension increases, the Q value of the cavity increases, and the size of the product also increases obviously; while when the cavity dimension decreases, the Q value of the cavity decreases, thereby obviously worsening an insertion loss index of the filter.
Therefore, a filter that does not affect the cavity quality factor and can implement the temperature compensation is needed.